1. Field of the Invention
The present invention relates to electromagnetic sensors. More specifically, the present invention relates to silicon microbolometers designed to sense infrared energy.
2. Description of the Related Art
Night vision systems are well known in the art. Night vision systems typically include a cryogenically cooled linear detector array with an associated cryogenic subsystem, a scanning system which moves the array across a two-dimensional field, and a diffractive optical system which focuses energy onto the detector. The detectors in the array either sense the heat of a body or detect low light levels.
While these systems have been used for military applications, the high cost of the scanning, cooling and optical systems associated therewith has heretofore limited the applicability of same for numerous other applications. U.S. patent application Ser. No. 08/232,893, filed Apr. 12, 1994, by S. H. Klapper et al., the teachings of which are incorporated herein by reference, discloses and claims a low cost camera for night vision systems including a focal plane array (FPA) of uncooled detectors and an optically fast, optical arrangement for focusing energy from an input aperture onto the array. The array includes a plurality of pyroelectric detectors which, in the illustrative embodiment, are fabricated of barium-strontium-titanate (BST) material.
While this system provides an inexpensive night vision system with good performance characteristics, there are a few areas in which the system may be further improved.
Firstly, the detector array of the above-described system requires a motor driven chopper wheel in front of the detector. The chopper wheel facilitates the sequential readout of the array and aids in the establishment of a DC reference level which is representative of the average DC level of the scene. Unfortunately, the chopper wheel and motor are expensive and fragile. In addition, the chopper wheel must be optically aligned and mechanically synchronized with the frame rate of the array.
Secondly, the barium-strontium-titanate detector requires temperature stabilization around a fixed reference point. Temperature stabilization consumes much power, requires recalibration and is tough to maintain in extreme environments.
Thirdly, the barium-strontium-titanate detector requires detector hybridization to the silicon readout chip. That is, the detector is of a different material composition than that of the silicon readout chip. The two types of material must be separately grown. This dictates that the two materials cannot be fabricated in the same facility and cannot be tested at the same test site. Each requires different plumbing, gas chamber, probe material and etc. An elaborate bump structure must be grown on the detector and the readout chip as the two chips are indium bump squeezed together. If the detector and the readout chip are not properly sandwiched together, the device will not work despite the fact that the detector and the chip were satisfactorily tested individually. Thus, the manufacturing yield is quite low. In addition, since the detector is opaque, there is no way of aligning it against the silicon. As a result, the hybrid detector requires much "hands on" fabrication. In short, the hybrid detector is costly to manufacture.
Fourthly, the BST detector is an inert ceramic material which does not react well to chemical etchants. Hence, a laser must be used to cut the detector elements. This creates a waffle construction. If the grooves between the detectors are not cut properly, cross-talk between detector elements results. Proper cutting with the laser creates small air gaps between the pixels. This leaves material of only 5-10 microns in thickness to hold the detector array together. This creates an extremely fragile construction which must endure the sandwiching operation described above. In addition, thermal isolation across the small junctions is imperfect. This creates a thermal bleedthrough or cross-talk which results in an electrical cross-talk.
Thus, other detector technologies have been considered. One alternative is to use a silicon microbolometer. A silicon microbolometer is a family of uncooled detector which sense heat based on a different principle than BST detectors. Unfortunately, although silicon microbolometers address many of the problems besetting BST detectors, in large uncooled focal plane arrays, silicon microbolometers suffer from excessive pixel to pixel nonuniformities due to the maturity of the detector process and large pedestals caused by the detector pulse bias mechanism. The nonuniformities are typically on the order of 5-10 times greater than the nonuniformities associated with BST detectors. This typically reduces the dynamic range available for the signal processing electronics. This has necessitated complex, high resolution, high performance, high cost, low yield signal processing electronics. Hence, the DC nonuniformities have heretofore eliminated silicon microbolometers from consideration for certain applications.
Thus, a need exists in the art for a system and technique for eliminating excessive pixel to pixel nonuniformity in large arrays of silicon microbolometer detectors.